1. Field of the Invention
The present invention relates to an image-forming apparatus such as a laser printer and a light scanning device mounted in the image-forming apparatus.
2. Description of the Related Art
A scanner unit for scanning a laser beam on a charged photoconductor to form an electrostatic latent image based on image data is mounted in an image-forming apparatus such as a laser printer.
Generally, the scanner unit includes a light source such as a semiconductor laser, a polygon mirror (rotational polygon mirror) driven to rotate by a motor, and an fθ lens having fθ characteristic. Light emitted from the light source is deflected by the polygon mirror and then an image of the light is formed on a charged surface of the photoconductor by the fθ lens.
In recent years, the demand for reduction in size of the scanner unit has increased with the advance of reduction in size of the image-forming apparatus. As an fθ lens provided in this type scanner unit, there has been proposed a system including an anamorphic plus lens which is formed so that at least one lens surface of the plus lens has a shape asymmetrically variable in the main scanning direction with respect to the center of the optical axis of the imaging device, and that the lens surface has a lens section symmetrically variable in the sub scanning direction with respect to the center of the optical axis of the imaging device to thereby correct asymmetry in curvature of field in the main scanning direction and the sub scanning direction (e.g. see Japanese Patent No. 3,035,993).
For example, there has been also proposed a system having a plurality of child line asymmetrical variable surfaces formed so that the curvature in the direction of a child line changes asymmetrically in the direction of a mother line in which the optical axis of the scanning optical device is sandwiched (e.g. see JP-A-2001-194611).
For example, there has been further proposed a system having two or more optical elements formed so that the two optical elements have at least one aspherical surface having positive refractive power in a main scanning section, and that the radii of curvature of at least two surfaces of the two optical elements have one extreme value along the main scanning direction in a sub scanning section (e.g. see JP-A-2003-156704).